1. Field of the Invention
The present invention relates to hydraulic shock absorbers, and more particularly to shock absorbers for use on the passenger cabin of an elevator or for the elevator counter-weights.
2. Description of the Prior Art
Shock absorbers of the above-mentioned type are devices which come into operation only under exceptional circumstances, viz. when, in an emergency, the passenger cabin or the counter-weights of an elevator fail to come to a halt at their lowermost normal stop. This may happen in the case of malfunction of the electrical controls, or in the case of material failure in the elevator suspension system. Emergency shock absorbers of this type are normally mounted on the bottom of the elevator shaft, using a vertically extending hydraulic cylinder and piston combination which, when engaged by the descending passenger cabin, moves against a progressively increasing hydraulic resistance.
In a known shock absorber of this type, the falling passenger cabin first contacts a compressible cushion on the upper end of the extended piston rod, whereupon the latter is forced downwardly into a cooperating cylinder from which a fluid column is displaced through a series of radial orifices in the cylinder wall. The displaced fluid moves into an expansion receptacle. The force with which the fluid column resists the penetration of the piston into the cylinder depends on the number and size of the radial orifices in the wall of the latter. The rate at which this resistance increases with increasing penetration of the piston is a function of the vertical spacing or gradation of these orifices. Consequently, the hydraulic pressure opposing the downward movement of the piston increases as the latter moves downwardly past some of the orifices in the cylinder wall, thereby reducing the number of remaining orifices through which the fluid can pass from the cylinder to the expansion receptacle. A return spring which normally holds the piston rod in its upwardly extended position, is at the same time compressed, but the resistance exerted by this spring is negligible in comparison to the hydraulic resistance in the cylinder. The purpose of this return spring is to automatically extend the shock absorber into its normal position, when the passenger cabin is raised from its overshot position.
The fact that this type of emergency hydraulic shock absorber never comes into operation under regular elevator usage brings with it the tendency that the inspection and servicing of these devices fall victim to neglect, with the risk that, when needed in an emergency situation, they fail to operate properly. This situation has led to officially enforced safety regulations which prescribe periodic inspection and servicing of all elevator shock absorbers, the regulations requiring that each shock absorber be equipped with either a fluid level sight glass or with a dip stick for the verification of the proper fluid level. Additionally, these regulations require that an electrical safety interlock be provided which assures that the shock absorber is always in its upwardly extended ready position.
The visual inspection of the proper fluid level in this type of shock absorber is frequently problematic, because shock absorbers are often mounted on the bottom of an elevator shaft which are difficult to reach for visual inspection. Additionally, fluid level sight glasses are difficult to inspect in the darkness and are readily covered with dust and grime. Fluid level inspection and, if necessary, a refilling of fluid to the required level in the sight glass may thus be very difficult. For this reason, it has become preferable to employ shock absorbers which are equipped with a dip stick. A known hydraulic shock absorber of the dip-stick-type features a vertical cylinder supported on a mounting base and communicating through a passage in that base with an expansion receptacle mounted on the same base. The cylinder and the expansion receptacle thus cooperate in the manner of U-shaped communicating vessels, the expansion receptacle having approximately the same height as the hydraulic cylinder. A dip stick reaching from above into the expansion receptacle serves to indicate the fluid level in the shock absorber.
Because of the need for two separate receptacles mounted on a common base plate and for communicating passages arranged in the latter, this known prior art device is comparatively costly to produce. It also requires a considerable amount of space in the elevator shaft.